Two-tone spectroscopy to characterize losses due to two-level systems*

It was shown that two-level system (TLS) defects are among the main sources of loss and decoherence in superconducting quantum devices. Microscopic nature of TLSs is still an open question and it is important to develop experimental techniques that allow to gather detailed information about them and characterize various materials beyond just the magnitude of the loss tangent. This knowledge will help to distinguish between various defects and loss mechanisms associated with them, and develop material processing and preparation procedures to improve coherence times of quantum computing devices.

We demostrate a two-tone spectroscopy technique to characterize the spectral line width of TLS. TLS spectral linewidth is related to the coupling of defects to phonon bath and can help to distinguishes various types of TLS defects.

We use the fundamental TM₀₁₀ pass band modes of 9-cell elliptical SRF cavities to probe the spectral properties of TLS in native niobium oxide. To achieve that, we pump one of the modes (4π/9 through 8π/9) with continuous wave signal to saturate TLSs that couple to this frequency, and we probe the Q-factor of the π-mode. We observe increase of the quality factor of the probed π-mode when one of the neighboring modes is pumped. Extremely high quality factors of the pass band modes and their relative proximity in the frequency space allow us to estimate the width of the spectral hole burned in the TLS bath absorption spectrum which is related to the line spectral width of the TLS defects.